This invention relates to the preflocculation of fillers used in papermaking, particularly, the production of shear resistant filler flocs with a defined and controllable size distribution at high filler solids is disclosed.
Increasing the filler content in printing and writing papers is of great interest for improving product quality as well as reducing raw material and energy costs. However, the substitution of cellulose fibers with fillers like calcium carbonate and clay reduces the strength of the finished sheet. Another problem when the filler content is increased is an increased difficulty of maintaining an even distribution of fillers across the three-dimensional sheet structure. An approach to reduce these negative effects of increasing filler content is to preflocculate fillers prior to their addition to the wet end approach system of the paper machine.
The definition of the term “preflocculation” is the modification of filler particles into agglomerates through treatment with coagulants and/or flocculants prior their flocculation and addition to the paper stock. The flocculation treatment and shear forces of the process determine the size distribution and stability of the flocs prior to addition to the paper stock. The chemical environment and high fluid shear rates present in modern high-speed papermaking require filler flocs to be stable and shear resistant. The floc size distribution provided by a preflocculation treatment should minimize the reduction of sheet strength with increased filler content, minimize the loss of optical efficiency from the filler particles, and minimize negative impacts on sheet uniformity and printability. Furthermore, the entire system must be economically feasible.
Therefore, the combination of high shear stability and sharp particle size distribution is vital to the success of filler preflocculation technology. However, filler flocs formed by a low molecular weight coagulant alone, including commonly used starch, tend to have a relatively small particle size that breaks down under the high shear forces of a paper machine. Filler flocs formed by a single high molecular weight flocculant tend to have a broad particle size distribution that is difficult to control, and the particle size distribution gets worse at higher filler solids levels, primarily due to the poor mixing of viscous flocculant solution into the slurry. Accordingly, there is an ongoing need for improved preflocculation technologies.
The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 C.F.R. §1.56(a) exists.